Recently, flat panel type displays (FPDs) such as liquid crystal displays (LCDs) and others, which can achieve high resolution, low power consumption and space saving have been extensively developed. Among these, application of LCDs for use in computer displays, television displays and others is quite significant. However, in contrast to the cathode ray tube (CRT) displays which used to be mainly adopted for these purposes, LCDs have been pointed out as a drawback which is so-called ‘blur injury’, that is, the edges of moving part are perceived to be hazy by the observer when a picture with motion is displayed.
As disclosed in, for example, Japanese Patent Application Laid-open Hei 9-325715, the cause of blur injury in motion picture display is not only attributed to the delay of the optical response time of liquid crystal, but also attributed to the LCD display method itself. CRT displays in which display is effected by illuminating the fluorescent body with scanning electronic beams, are of so-called impulse-type display in which emission of light from each pixel presents a generally impulsive characteristic though a slight afterglow of the fluorescent body may occur.
In contrast, because the electricity charged by application of an electric field to the liquid crystal is held at relatively high ratio until the next application of an electric field (in particular, TFT LCDs present a remarkably high charge holding performance because every dot that constitutes a pixel is formed with a TFT switch and every pixel normally has sub capacitance), LCD displays are driven in a so-called hold-type display mode in which each liquid crystal pixel continues to emit light until data is rewritten by application of an electric field based on the image data of the next frame.
In such a hold-type display, the impulse response of image display light has a temporal spread, hence the temporal frequency characteristic lowers, which in turn causes degradation of the spatial frequency characteristic, leading to blur in the observed image. To deal with this, the above-mentioned Japanese Patent Application Laid-open Hei 9-325715 has proposed a display device improved in blur injury in the observed image, by on-off controlling a shutter disposed over the display surface or a light source lamp (backlight) so that the display light will be presented to the observer during only the rear half period of each field of the display image, to thereby limit the temporal spread of impulse response.
This will be detailed with reference to FIGS. 1 and 2. In FIG. 1, 111 designates a light source lamp such as a strobe, etc., which can be turned on and off at high speed; 112 a power source for supplying electric power to light source lamp 111; 113 a transmission type display device such as a TFT liquid crystal device, etc., which converts electric image signals into light for image display; 116 a drive circuit for generating drive signals for driving display device 113 in accordance with the image signals and synchronizing signals; and 117 a pulse generating circuit for generating control pulses in synchronism with the vertical synchronization of the input synchronizing signals so as to perform on/off control of power source 112.
When the illumination ratio is 50%, light source lamp 111 is turned off during the period from time t1 to time t2 within one field period T and turned on during the period from time t2 to time t3, by pulsing application of electric power from power source 112 (see FIG. 2). When the illumination ratio is 25%, the lamp is turned off during the period from time t1 to time t6 within one field period T and turned on during the period from time t6 to time t3, by pulsing application of electric power from power source 112 (see FIG. 2).
In sum, the illuminating period of light source lamp 111 is controlled by pulse generating circuit 117 and power source 112. Accordingly, total response of image display light for image display is given by the pulse-on waveform from time t2 to time t3 and the pulse-on waveform from time t4 to t5 only, for the case of the illumination ratio of 50%, for instance. Therefore, the temporal spread of total response for display is reduced and the temporal frequency characteristic is also improved to be flatter, so that image quality degradation during displaying motion pictures can be inhibited.
The technique for suppressing image quality degradation such as blur injury, etc., arising when displaying motion pictures, by illuminating the full screen range with the backlight a predetermined time after data writing of the image signal for one frame to be displayed on the LCD panel is called a full-screen flashing type, which has been also disclosed in, for instance, Japanese Patent Application Laid-open 2001-201763, Japanese Patent Application Laid-open 2002-55657 and others, other than the above-mentioned Japanese Patent Application Laid-open Hei 9-325715.
In contrast to this full-screen flashing type backlighting technique, so-called scanning type backlighting schemes have been proposed in, for instance, Japanese Patent Application Laid-open Hei 11-202286, Japanese Patent Application Laid-open 2000-321551, Japanese Patent Application Laid-open 2001-296838, in which image quality degradation such as blur injury etc., arising during displaying motion pictures, is suppressed by sequentially activating scan-wise multiple backlight for divided lighting areas that correspond to multiple divided display areas of the LCD panel.
The configuration which approximates impulse-type drive display such as a CRT, from hold-type drive display by high-speed sequential flashing of backlight will be described with reference to FIGS. 3 to 5. In FIG. 3, a multiple number of (four, in this case) direct fluorescent lamps (CCFT) 203 to 206 are arranged parallel to the scan lines, on the backside of a liquid crystal display panel 202, and the lamps 203 to 206 are sequentially activated from top to bottom, in synchronization with the scan signals for liquid crystal display panel 202. Here, lamps 203 to 206 correspond to four display areas into which liquid crystal display panel 202 is divided in the horizontal direction.
FIG. 4 is a chart showing activation timing of the lamps corresponding to FIG. 3. In FIG. 4, the high state presents the lighted state of the lamp. For example, the video signal is written into the top one-fourth of the display area of liquid crystal display panel 202, in duration (1) within one frame period, and fluorescent lamp 203 is activated in duration (4) after a delay of durations (2) and (3) for liquid crystal response time. In this way, the lamps for divided display areas are repeatedly and sequentially activated within one frame period by one after another after writing of the video signal.
Thereby it is possible to simulate impulse-type drive display of a CRT, from hold-type drive display of an LCD, hence the video signal of the previous frame is not perceived when a motion picture is displayed. Consequently it is possible to prevent degradation of motion picture display quality due to edge blur. It should be noted that the same effect can be obtained by activating two lamps at the same time as shown in FIG. 5. This method also lengthen the lit time of the backlight, so that it is possible to prevent decrease of backlight brightness.
Further, in this scan-type backlighting technique, for each of the multiply divided display areas of the liquid crystal display panel, the luminous area corresponding to the backlight is illuminated at a timing when the liquid crystal has been brought to a full optical response, the duration from the time the image is written into the liquid crystal to the time the backlight is activated can be made equal regardless of the position (vertical position) on the display screen. As a result, this configuration is advantageous in making satisfactory improvement of blur injury in motion pictures regardless of the position on the display screen.
On the other hand, contrasting to the above intermittent backlight driving scheme, there have been proposed so-called black insert type liquid crystal displays, in which, instead of making intermittent the backlight in one frame period, the video signal and the black signal are alternately written into the liquid crystal display panel in one frame period so that the light emission time of each pixel (image display duration) from the time a certain video signal frame is scanned to the time the next frame is scanned is shortened to realize emulative impulse type display.
Known examples of such black insert type liquid crystal displays include: one in which, as shown in FIG. 6(a), one frame of input image data is sequentially written into the liquid crystal display panel, then the whole screen is written in with black display data so that the display of the whole screen is blackened in a predetermined period; and one in which, as shown in FIG. 6(b), part of the screen is displayed with black for a predetermined period so as to shorten the span for displaying the image in one frame period compared to the conventional hold-type display device, by sequentially writing black display data every scan line (Japanese Patent Application Laid-open Hei 9-127917 and Japanese Patent Application Laid-open Hei 11-109921).
In the above-described conventional technologies, attempts to amend image quality degradation due to blur injury arising when displaying motion pictures in a hold-type display device, are made to simulate impulse-type drive display drive as in a CRT or the like, from hold-type drive display drive, by shortening the span of image display, specifically, by implementing intermittent backlight drive within one frame period (e.g., 16.7 msec in the case of 60 Hz progressive scan), or by writing black display data to the liquid crystal display panel after writing of image display data.
Here, in order to amend image quality degradation due to blur injury, it is preferred that the impulse ratio (the ratio of the image display duration in one frame period) is made lower. However, reduction of the impulse ratio may induce the following problems (1) to (3).    (1) The extent of the effect of motion blur depends on the image type. For example, in the case of CG (computer graphics), animation and game images, the movie is rendered by a series discrete images (at one moment only within every frame) as shown in FIG. 7(a) though they are supposed to be continuous. That is, there are some cases where no motion blur which will function to interpolate interval between frames is added.
Smooth motion can be obtained if motion blur is generated and added by an image process. However, when a picture without any motion blur, i.e., a content image which originally lacks smoothness in motion is displayed with a low impulse ratio, a stroboscopic defect, i.e., discrete motion of moving objects, occurs, leading to more trouble of image quality degradation.
Images taken by a storage type camera that is usually used as a television camera, have different amounts of motion blur depending on the shutter speeds, because each frame is an accumulation of light while the shutter being open. For example, since movies and images taken indoors such as in a studio under strong lighting (e.g., news programs, broadcasts of indoor competitions such as swimming races) are taken at high shutter speeds (that is, the opening duration of the shutter is short), a moving object is supposed to be added with a small amount of motion blur during shooting, as shown in FIG. 7(b). When such an image with a small amount of motion blur is displayed with a low impulse ratio, there is a high possibility of the aforementioned stroboscopic defect occurring.
On the other hand, an image that is shot dark, outdoors, such as a broadcast of a night game of baseball match or soccer match may be taken at low shutter speeds (that is, the opening duration of the shutter is long). In such a case, a moving object is supposed to be added with a large amount of motion blur during shooting, as shown in FIG. 7(c). When such an image with a large amount of motion blur is displayed with a low impulse ratio, smooth motion can be reproduced by virtue of motion blur. Thus, in this case no stroboscopic defect stated above will occur, therefore it is preferred to give priority to display of a sharp and clear motion picture by reducing blur injury.    (2) Secondly, the visual characteristics when watching motion pictures is considered to be attributed to ocular movement, time integration of vision and the non-linearity of the visual response to photic stimulation intensity. Of ocular movement, the characteristic of the following movement (movement of left and right eyes chasing a moving object approximately similarly), which is the most important characteristic for perceiving motion pictures, varies depending on the speeds of moving objects and the like, and there is a possibility that the aforementioned stroboscopic defect may occur in some image contents when the image is displayed with a low impulse ratio.
For example, in the case of an image (motion pan), i.e., where the full frame uniformly moves in the horizontal direction such as in a sport program broadcast of a soccer or volleyball game, it is preferred that sharp and clear display of motion pictures reduced in blur injury is achieved by setting the impulse ratio as low as possible because image quality degradation due to blur injury becomes conspicuous. In contrast, when a target person is fixed with the background being moved, there is a high risk of image quality degradation due to occurrence of the aforementioned stroboscopic defect if the impulse ratio is set low.    (3) Further, if the impulse ratio is set low, it is true motion picture blur injury defects will be reduced. However, because black display duration (non-image display duration) in one frame period increases, flicker becomes conspicuous especially in white image display areas and leads to image degradation due to flickering.
As has been described above, when the impulse ratio is set low, image quality may degrade due to occurrence of stroboscopic, flickering or other image quality defects depending on the type of image content, hence it has been difficult to achieve improvement of total image quality.
Further, the optimal impulse ratio is different depending on the image contents, image materials and the like. Moreover, sensitivity (dynamic visual acuity) to blur injury, stroboscopic effects and flickering greatly varies between individual users, so that it is impossible to realize improvement of total image quality for individual users.
In view of the above problems, it is therefore an object of the present invention to provide a liquid crystal display which can realize improvement of total image quality, by variably controlling the ratio of the image display duration in one frame period in accordance with the type of the image content to be displayed so as to suitably suppress the image quality degradation due to blur injury, stroboscopic effect, flickering and other defects.
Also, in view of the above problems, it is another object of the present invention to provide a liquid crystal display which can realize improvement of total image quality for individual users, by allowing for variable control of the ratio of the image display duration in one frame period in accordance with the user's instructional input so as to suitably suppress the image quality degradation due to blur injury, stroboscopic effect, flickering and other defects.